Caption: Nuclear reactor core glowing with Cerenkov radiation. Cerenkov radiation is light emitted when a charged particle travels through a material at a higher speed than light can travel in that medium. Although nothing can travel faster than light in a vacuum, in water, for instance, light only travels at three-quarters of its vacuum speed. This means electrons and other particles involved in nuclear reactions can be accelerated above local light speed. The passage of the particle polarises the atoms in the medium, which emit photons as they return to equilibrium. This is a reactor at the Research Institute of Atomic Reactors, at Dimitrovgrad, in Russia. It is being used to produce the radio-isotope Molybdenum-99, which decays over a few days to the short-lived isotope Technetium-99m, which is valuable in medical imaging.